<!ELEMENT geology (volcano)+>
<!ELEMENT volcano (location,height,type,eruption+,magma,comment?)>
<!ATTLIST volcano name CDATA #IMPLIED>
<!ELEMENT location (#PCDATA)>
<!ELEMENT height EMPTY>
<!ATTLIST height value CDATA #IMPLIED
                 unit CDATA #IMPLIED>
<!ELEMENT type (#PCDATA)>
<!ELEMENT eruption (#PCDATA)>
<!ELEMENT magma (#PCDATA)>
<!ELEMENT comment (#PCDATA)>