51 



aggregated into a single mass, would form a body whose specific gravity 

 would probably lie between 4'5 and 5'7, so long as its dimensions were 

 moderate." The most reliable estimate of the average specific gravity of 

 the earth is 5 '6, and it is thus seen that there is a very close agreement 

 between the specific gravity of the earth, regarded as a whole, and that of 

 the ideal meteoric mass ; an agreement which becomes still closer if we take 

 into consideration the probable effect of gravitation in increasing the density 

 of the central portions in a mass of the size of the earth. 



A consideration therefore of all the facts known with regard to the 

 specific gravity of terrestrial and extra-terrestrial rocks, as well as of the 

 earth itself, points to the conclusion that the interior portions of the planet 

 are largely composed of metallic substances. 



TEXTURE. The texture of a rock depends on the shape, size, physical 

 condition and mode of arrangement of the individual constituents. The term 

 structure is sometimes used in the same sense, but it is more frequently 

 employed with reference to the behaviour of rocks in large masses. 



When an igneous rock contains a number of more or less spherical or 

 elliptical cavities, due to the escape of gases and vapours during the process 

 of cooling, it is said to be vesicular. When the cavities are very numerous, 

 as in pumice, it is said to be pumiceous or scoriaceous. When the cavities 

 have been filled up with secondary minerals the rock becomes amygdaloidal. 

 In certain cases cavities, occupied by secondary minerals, have been produced 

 by the partial decomposition of an originally homogeneous rock. Such 

 cavities are termed pseudo-amygdaloids. They may be distinguished from 

 true amygdaloids, at any rate in most cases, by their irregularity in form. 



Rocks which were solid throughout when first formed present a number 

 of important modifications of texture. Such a rock may be composed entirely 

 of crystals or crystalline grains, in which case it is said to be holo crystalline; 

 or it may be composed partly of crystalline and partly of amorphous matter, 

 in which case it is said to be semi-crystalline; or, again, it may consist almost 

 entirely of glass, in which case it is said to be glassy or vitreous. In the 

 holocrystalline rocks two important varieties of texture, depending on the 

 relative sizes of the constituents, are recognised. When the constituents are 

 of tolerably uniform size, and intimately intercrystallised, the rock is said to be 

 granular. When, on the other hand, certain constituents occur as large and 

 more or less perfect crystals in a matrix of finer grain, the rock is said to be 

 porphyritic. The rocks which are composed partly of crystalline and partly 

 of amorphous matter are usually regarded as porphyritic. The essential 

 feature of the porphyritic texture, as thus defined, is the occurrence of certain 

 crystalline constituents which are differentiated from those forming the mass 

 of the rock, the groundmass or matrix, by reason of their size. 



When the porphyritic texture can be recognised with the naked eye the 

 rock is said to be macro-porpliyritic : when it can only be made out distinctly 

 by the use of the microscope it is described as micro-porphyritic. 



Prof essorRosENBUSCH has proposed that the above use of the term porphyritic 

 should be dropped, and its meaning so extended as to include certain rocks in 

 which the crystalline constituents are of tolerably uniform dimensions, but 



