26 THE CANADIAN NATURALIST. [Feb, 



spathic rocks, has now become so extended that it is to be regarded 

 as only indicating an accident of structure. The title of amyg- 

 daloid is given to various rocks having rounded cavities which are 

 wholly or partially filled with various crystalline minerals. The 

 base of these rocks is generally granular or crypto-crystalline; 

 but is sometimes amorphous, resembling a scoria or vesicular 

 lava, the cavities of which have been filled by infiltration. Such is 

 doubtless the origin of some amygdaloids. In more cases how- 

 ever these cavities have probably been formed like those often 

 found in dolomites, and in some other rocks, by a contraction during 

 solidification. Porphyroid rocks, in which quartz, orthoclase, and 

 other mmerals are arranged in orbicular masses, are also sometimes 

 designated as amygdaloids, and may be confounded with the 

 two previous classes in which the imbedded minerals are the result 

 of subsequent infiltration. Allied in strucure and origin to the- 

 last are what are named variolites or variolitic rocks. (See 

 Geology of Canada, pp. 606, 607.) 



The masses into which some aluminous minerals enter as a 

 prominent element constitute by far the greater part of the rocks 

 now under consideration. These are naturally divided into two 

 classes, whose origin we have pointed out in a recent paper already 

 referred to. (Silliman's Journal [2], xxxvi, 218.) The first of these is 

 characterized by containing an excess of silica, with a portion of alu- 

 mina, much potash, and small portions only of lime, magnesia, and 

 oxyd of iron. The second class contains a smaller amount of silica, 

 and larger proportions of alumina, lime, magnesia, and oxyd of iron, 

 with soda, and but little potash. These chemical differences are 

 made apparent in the more coarsely crystalline rocks, by the nature 

 of the constituent minerals ; and in the compact varieties, by diff"er- 

 ences in color, specific gravity, and hardness. Thus in the rocks of 

 the first class the predominant mineral is orthoclase, generally asso- 

 ciated with quartz, and the composite rocks of this class seldom 

 have a density much above that of these species; or from 2.6 to 2.7.. 

 In the second class, the characteristic mineral is atriclinic feldspar, 

 with pyroxene or hornblende, the feldspar sometimes predominant ; 

 while in other cases the pyroxene or hornblende makes up the 

 principal part of the rock. The presence of these latter minerals 

 generally gives to the fine-grained rocks of this class a dark color,, 

 a hardness somewhat inferior to the more silicious class, and a 

 density which may vary from 2.7 to more than 3.0. It will 



