A DEVICE FOR IDENTIFICATION OF BLAST HOLES THAT EXCEED A PRE DEFINED "HOT HOLE" TEMPERATURE FIELD OF THE INVENTION The invention relates to the mining and is particularly but not limited to the measurement of blast hole temperatures that exceed a pre determined "hot hole" temperature concerned with drilling and blasting operations. BACKGROUND TO THE INVENTION The use of explosives to break rock is a major activity in mining operations. In open cut mining the process will involve drilling many hundreds of boreholes and loading them with explosives that are appropriate for ground conditions and the desired blast pattern. In some instances the temperature in the blast holes can be greater than 55 0 C due to heating. The cause of the heating includes burning coal and sulphide oxidation or geothermal heating in underground mining. In the case where the temperature inside the blast hole is greater than 550C the particular hole is identified as a "hot hole" and a specific blasting regime is required when loading this hole with explosives. If a "hot hole" is not identified and loaded according to the required loading practice, the hole may spontaneously detonate with often disastrous consequences. 1 Blast holes are usually drilled in patterns of many hundreds of holes and the occurrence of a "hot hole" is somewhat random because of the lack of detailed knowledge of below ground conditions. If a there is a history of "hot holes" at a mine site then each hole is normally subjected to a temperature scan. The scan involves lowering an electronic temperature transducer down a hole via a cable and reading the temperature on a display at the surface. In some cases when temperature measurements of the holes are not taken, the blasting personal may identify hot holes during hole depth measurement operations. After the drilling of blast holes has been completed, the hole depth measurement is performed by feeding a measuring line into the hole. For blast holes depth measurement it is normal practice to use a tape measure with a weight attached to the end. Both total hole depth and water level depth if present are recorded in addition to the hole identification number. The process in known as "dipping" and is a two or more person task with as many as two thousand holes being dipped for a single blast. One or more persons operate the tape measure and another person records the measurements. This information is used to determine if the hole has collapsed and the type of explosives to be used in the particular hole. In the case of identifying "hot holes" the operator will observe that the weight is at a higher than normal temperature and then advise that a temperature scan of the hole is required. As the prior art procedure requires a number of operators and temperature sensing equipment there is the capacity for both equipment and human error including increased costs. 2 It is therefore an object of the invention to seek to ameliorate or eliminate the disadvantages or limitations of the prior art by providing an apparatus and method of rapidly identifying hot holes during hole dipping operations that is cost effective and fail safe. STATEMENT OF THE INVENTION In one aspect, the invention resides in an apparatus for rapidly identifying high temperature blast holes or "hot holes" including in combination: a temperature reactive tag adapted to change colour; a cord to lower the tag into and retrieve the tag from a blast hole; the tag having a low mass and an outer configuration wherein the tag is rapidly heated by the blast hole temperature, and wherein the colour of the tag is non reversibly changed when the tag is exposed to a temperature that exceeds a predetermined temperature. Preferably, the tag comprises a plastic card or region of the card coated with a heat sensitive chemical that permanently changes colour when exposed to a temperature that exceeds a predetermined temperature. In a preferred example, there can be a non-reversible temperature indicator that is attached to the plastic card which is at an edge of the card. 3 The heat sensitive chemical is preferably isolated from the rest of the card by slots to enhance thermal isolation and decrease the time required for the chemical to reach the surrounding temperature. Preferably, the pre determined temperature at which the tag changes colour may be 550C, 57 0 C, 65"C or other temperature according to particular rules and regulations for the identification of blast holes as hot holes. In another example, the tag may be comprised of a plurality of regions which change colour at different temperatures. Preferably, the tags have pre-cut holes and slits to allow rapid attachment and removal from a tape measure or cord that comprises part of a blast hole depth measuring apparatus wherein the depth to which the tag has been lowered is recorded. Preferably, there is a global positioning system (GPS) associated with the depth measuring apparatus to identify the location of the blast hole. Preferably, there is computer recordal means to record the GPS position and depth of the blast hole identified as a hot hole. In another aspect the invention resides in a method of rapidly identifying and logging high temperature blast holes or "hot holes "using the apparatus hereinabove described including the steps of: 4 (a) lowering a tag with a cord into the blast hole; (b) measuring the length of the cord as a measurement of depth of the blast hole; (c) retrieving the tag with the cord; (d) examining the tag for a change of colour. (e) if there is a colour change, recording the depth at which the tag was lowered in the blast hole, together with the GPS location of the identified hot hole on computerised or other data logging means. Preferably, the method is conducted as part of hole- dipper operations wherein a weight is attached to a tape measure and dropped down the blast hole to measure the depth. Preferably, the disposable plastic cards have pre-cut holes and slits to allow rapid attachment and removal from the tape measure. According to the present invention the method provides for identifying a blast hole as a "hot hole" during hole- dipping operations. In hole- dipper operations a weight is attached to a tape measure and dropped down the blast hole to measure the depth. The method involves a disposable plastic card that is attached to the tape measure immediately above the weight for a single point measurement. In an alternative example, multiple disposable plastic cards can be attached along the length of the tape measure. The disposable plastic cards have pre-cut holes and slits to allow rapid attachment and removal from the tape measure. The cards are dark in colour and are of low mass to allow them to rapidly acquire the temperature of the surrounding conditions. The card contains a non-reversible temperature indicator which changes colour when subjected to the temperature 5 that is deemed to indicate the hole is at a temperature that it will be classified as a "hot hole". The area of attachment of the temperature indicator is at the edge of the card and is isolated on two sides by slots. This arrangement enhances the thermal isolation of the area which decreases the time required for the area to reach the surrounding temperature. The temperature may be 550C, 570C, 65 0 C or some other temperature according to particular rules and regulations. The operator will observe the non-reversible temperature indicator and note that the hole is classified as a "hot hole" if any of the temperature indicators change colour. The plastic card device is replaced once it has changed colour. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention be better understood and put into practise, reference is made to the accompanying drawings wherein: Figure 1 shows a plastic card containing the non-reversible temperature indicator according to the invention. DETAILED DESCRIPTION OF THE DRAWINGS A more detailed description will now be provided with reference to the accompanying drawings. It will be appreciated that the detailed description is intended to provide details of preferred features of the invention and should not be construed as limiting on the invention in any way. Figure 1 shows the plastic card 10 containing the non-reversible heat sensitive chemical temperature indicator 12. The slits 14,16,18,20 and holes 24,26,28,30 are 6 used to rapidly attach and detach the card from a cord, cable, line or tape measure (not shown). The card is preferably dark in colour and of low mass to allow for rapid heat absorption to the temperature surrounding the card. The area of attachment of the temperature indicator 12 is at the edge of the card and is isolated on two sides by slots 32, 34. This arrangement enhances the thermal isolation of the heat sensitive chemical area which decreases the time required for the chemical to reach the surrounding temperature. The non-reversible temperature indicator changes colour when subject to a pre determined requisite temperature. The particular temperature is fixed and will normally be 55'C, 570C or 650C as these are the most common temperatures that have been mandated as the threshold for the identification of "hot holes". It will of course be realized that while the foregoing has been given by way of illustrative example of this invention, all such and other modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of this invention as is herein set forth. In the specification the terms "comprising" and "containing" shall be understood to have a broad meaning similar to the term "including" and will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. This definition also applies to variations on the terms "comprising" and "containing" such as "comprise", "comprises", "contain" and "contains".