THE CHIEF ZEOLITES. 



29 



Professor Daubree notices tliat not only do many of these zeolites 

 occur in the same mass of rock, but that several occur together in the 

 same cavity. There is every reason to believe that they owe their 

 existence to the infiltration of ordinary water, because they have been 

 found formed in the brickwork of several old Roman baths, as at 

 Plombieres in the Vosges, where the warm water had contained 

 alkaline substances. The nature of the rock in which it is deposited 

 appears to exercise some influence on the kind of zeolite formed, 

 because the species in the mortar are different to those which occur 

 in the bricks. 



It may, perhaps, be imagined that innumerable mineral combina- 

 tions are derived from the sixty-six primary chemical elements. But, 

 as many of them are excessively rare, as the remainder combine only 

 upon certain principles, the number of mineral species really deter- 

 mined is, in fact, not large, perhaps hardly exceeding five hundred. 

 Nor is the geologist often called upon to make himself acquainted 

 with all even of this moderate number. Unless his labours are 

 devoted to the detailed phenomena of volcanic productions or of 

 mineral veins, he will seldom have occasion to observe more than ond- 

 tenth of the number. The reason of this is that a large portion con- 

 sists of rare and local species ; and that, in combining to form rocks, 

 the others are associated in families, and united into specific com- 

 pounds without much permutation. In consequence, there is really 

 less difficulty than might be expected in recognising and discrimi- 

 nating the rocks. To class and to describe them in a true natural 

 order is difficult, to compare and to know them according to their 

 mode of occurrence is easy, and should form part of the practical 

 education of every geologist. The student should lose no time in 

 obtaining slices of the chief rock-forming minerals, and must study 

 'iem under the microscope with and without polarised light, if he 

 mid learn to discriminate the varieties of igneous rocks which they 



ibine to form. 



