70 Dr. Mac CuUoch on Jasper. 



equally frangible in every direction, unless when it possesses 

 some peculiar internal structure. Although the fracture is 

 generally minutely granular, and the surface arid, it is occa- 

 sionally also more or less splintery, or even conchoidal ; 

 while the minuter fragments may also be sometimes translucent. 

 It is not easily confounded with any rocks except the indurated 

 claystones and the pitchstones. From the intimate nature of 

 their connexions it is obvious that it cannot ahvays be dis- 

 tinguished from the former. From the latter it is more easily 

 discriminated, by the more frequent absence of the resinous 

 lustre and of the peculiar transparency of the fragments ; although 

 even pitchstone sometimes puts on characters by which it oc- 

 casionally approximates to jasper. If nature has not always 

 created definite boundaries, it is in vain for mineralogists to at- 

 tempt it. 



The colours of jasper are infinitely various, and are the prin- 

 cipal cause of its estimation among mineralogists and lapidaries. 

 They are also, in general, much more brilliant and decided 

 than those of any other rock except limestone ; yet the student 

 will, from the preceding description, beware of using them as an 

 empirical character to the neglect of others. Red of various 

 hues, ochre yellow, greens, browns, greys of all tones, and 

 black, are the prevailing tints ; and they occur in every mode 

 of intermixture, so as to present almost infinite varieties. From 

 a wish to conform to the popular practice respecting this rock, 

 of which the mere mineral characters are not much varied, these 

 distinctions have, therefore, been introduced into the synopsis 

 in a more conspicuous manner than has been adopted with regard 

 to any other of the substances which I have attempted to render 

 intelligible to beginners by a synoptical detail of varieties. 



Synopsis of Jasper. 



A. With a dull earthy fracture, and passing into claystone, of 

 which it appears to be a modif cation. 



This varies much in colour, but the term is generally limited 

 to those varieties which possess decided or brilliant hues. Reds 



