278 HISTORY OF GEOLOGY AND PALAEONTOLOGY. 



seated batholites, as laccolites intruded at various horizons 

 between the sedimentary deposits, as fissure eruptions, or 

 volcanic explosions. He summarises his views in the 

 following sentences: "The uppermost peripheral parts of the 

 earth's body are held firmly arched in virtue of the tangential 

 tensions. Either radial tensions or crust sagging causes a part 

 of the earth's body to split away from the outer crust towards 

 the interior, and a large cavity or macula forms more or less 

 parallel with the earth's surface, lenticular in shape if produced 

 mainly by sagging, and wider if due to radial fracture. The 

 macula fills with lava ; and if the surface rocks subjected to 

 tangential tensions find escape from them in any direction, for 

 instance by a folding movement or by the overthrust of 

 another mass of rock, then the relieved portion of the arched 

 crust which is immediately above the macula sinks into it and 

 lava wells forth at the faults and deeper inthrows " (loc. cit., 

 vol. i., p. 220). 



Dr. Reyer, in his work Theoretische. Geologic (1888), 

 groups batholites, laccolites, domes (Kuppen), and sheets as 

 massive eruptions, and distinguishes them from true volcanic 

 eruptions associated with fragmentary discharges. At the 

 same time he allows that in Mexico, Iceland, and in other 

 localities, massive intrusions and outpourings occur in 

 combination with typical tuff volcanoes. Reyer contests 

 Gilbert's explanation of laccolites as intrusions following the 

 bedding-planes of strata ; he regards them primarily as surface 

 protrusions contemporaneous with the sedimentary deposits in 

 association with which they occur; and with regard to the 

 apophyses extending from laccolitic invasions into super- 

 incumbent strata, Reyer says they are intrusions altogether 

 subsequent to the laccolites. True volcanic mountains must, 

 according to Reyer, include tuffs and loose fragmental 

 products, but may or may not include lava ; these are piled 

 round the orifice and arranged as inclined successive layers. 

 The craters are, he thinks, usually the result of explosion ; 

 occasionally, however, they arise from inthrow. The larger 

 areas of subsidence, on which the volcanic mountains are 

 found, appear to have been formed by repeated eruptions. 



It had been recognised by Dolomieu and Spallanzani that 

 the violent outbursts from active volcanoes could not be 

 entirely due to the pressure of the outer firm envelope of the 

 earth upon internal molten material. But, whereas Dolomieu 



