Primary Sandstone. — Quartz Rock. 315 



by exposure to air, and this effect penetrating to an inch or more 

 in depth. It will be an interesting object tor chemists versant 

 in the analysis of minerals, to inquire respecting the nature of 

 this colouring matter ; a circumstance as yet very obscure in all 

 the varieties of fluor spar. It presents an analogy to the topaz, 

 of which the yellow colour is dis.^ipated by a low heat without 

 altering the transparency of the mineral ; and, instead of arising 

 from chemical composition, may possil)ly be of an optical nature; 

 as by the loss of water, or other causes, the arrangement of the 

 particles that determine the reflection of the green rays may be 

 changed. 



PRIMARY SANDSTONE. 



We are indebted to Dr. MacCulloch for the first discovery of 

 this important rock, which had been either unobserved, or con- 

 founded with the secondary red sandstone; the old red sandstone 

 of the Wernerian system. The first account of it was contained 

 in his ]}apers on the Isle of Sky, published in the Geological 

 Transactions, and, in these, its geological characters, and its 

 claims to a place in the class of primary rocks, were satisfacto- 

 rily proved. We understand that a fuller account of it will be 

 contained in his work on the Western Islands of Scotland, now 

 on the eve of publication ; where he has shown that it occupies 

 an extensive district on the western coast. We are sorry, how- 

 ever, to see that the map which he has given, in which that rock 

 appears, (of which we have seen the proofs in the publisher's 

 hands,) does not contain the whole of that coast, as well as the 

 islands ; having been curtailed to accommodate the size of the 

 volume. We wait with impatience for these details, as the dis- 

 covery of a sandstone and a conglomerate alternating with gneiss, 

 must form an important revolution both in the theories of our 

 geologists, and in the science of geology. 



QUARTZ ROCK. 



It is to the same industrious geologist that we are indebted 

 for having first ascertained the true characters of this rock in all 

 its varieties, and the several connexions under which it appears 

 among the primary strata, to which it belongs. His predecessors 

 were at a loss what to do with it in their systems ; nor had they 

 observed the extent of its range, or the nature and variety of its 

 connexions. His papers on this snljject were printed in the 

 Geological Transactions, apparently, jutt as the several facts had 

 occurred to him in succession. As he has there described it 

 forming the island of Jura, we expect to find a fuller detail of it 

 in the work above mentioned, where that island is described. 



It appears, from those accounts which we have read, to oc- 

 cupy a considerable .sjjacc in Scotland ; occurring in alternation 



with 



